This section is intended to introduce various aspects of the art and provide a framework to facilitate a better understanding of particular aspects of the present invention. While some aspects of the discussion in this section may be associated with exemplary embodiments of the present invention, the section should not be read necessarily as admissions of prior art.
Illitization (direct illite precipitation or smectite to illite conversion) is a widely used tracer for evaluating thermal evolution in volcano-sedimentary sequences during burial, metamorphic and tectonic events. Studies of light element isotopic geochemistry of illite-type nanocrystals have greatly improved our understanding of the illitization process (see Clauer, et. al., Chemical Geology 383 (2014): 26-50, which is incorporated by reference herein), and light element isotopes, such as δ7Li and δ11B, have been shown to record specific paleofluid changes during illite crystal growth (see Williams, et. al., Geochimica et Cosmochimica Acta 120 (2013): 582-599, which is incorporated by reference herein).
In illite-smectite (I-S), Li substitutes in octahedral sites and B substitutes in tetrahedral sites (see FIG. 1A). In smectite, B and Li may also reside in the interlayers; however, the interlayer adsorbed contaminants may be removed to obtain consistent results for I-L. Over the past decade, fractionation curves between illite and water have been determined empirically, experimentally and theoretically for both lithium (FIG. 1B) and boron (FIG. 1C) isotopic fractionation.
The production of oil from shales has risen dramatically since 2010, when the price of oil reached $100/barrel. A particularly rich source of oil is the lower Bakken shale, which extends from western North Dakota into eastern Montana and up north into Canada. Lower Bakken shale is primarily illitic (72% illite-smectite), but also includes other components, such as quartz (15%), pyrite (7%), K-feldspar (3%), dolomite (2%), and plagioclase (1%).
Several regions of the Bakken shale are rich in organic compounds. In addition to the mineral components, such oil shales contain kerogen, a matrix made of largely insoluble, high molecular weight hydrocarbons, and bitumen, which is the source of the lower molecular weight organic solvent-soluble hydrocarbons that make up shale oil.
The oil producing potential of oil shale increases as the kerogen matrix thermally matures from the immature to the mature state (peak oil), and then decreases as the kerogen advances to an overmature and unproductive state. However, in some cases, an overmature kerogen matrix may host a substantial reservoir of extractable and commercially valuable hydrocarbons. Because of the thermal maturity of the kerogen matrix, such hydrocarbons are likely not sourced from the host rock, but have likely migrated from non-local bitumens.
Optimizing resource recovery while minimizing environmental impact are primary goals when extracting oil from oil shales. To identify optimal targets for enhanced oil recovery, it would be helpful to be able to accurately identify the source of hydrocarbons residing in the pores of a given host rock. For example, if the hydrocarbons in the pores of an overmature kerogen matrix can be shown to come from a mature (i.e., peak oil) source, extraction of the hydrocarbons may be commercially worthwhile, despite the overmature (i.e., unproductive) status of the host rock. Furthermore, accurate identification of the source of hydrocarbon contamination of groundwater can be used to monitor and remediate such contamination.
Conventional methods for linking bitumen and other hydrocarbon byproducts to their source rock include using a variety of organic biomarkers as source tracers. Many of these biomarkers are redox-sensitive, and tracing them often requires specialized and complex organic chemical analyses. Thus, there is a need in the art for a method of tracing hydrocarbons in a host rock to the original source rock that is not redox-sensitive, and that is independent of any organic compounds present in the hydrocarbons. Such a method could be used to identify areas for most productive hydraulic fracturing, and to promote environmentally responsible production of oil and gas.